1. Field of the Invention
The present invention relates to a DC/DC converter which limits a flyback voltage (reset voltage) generated in a primary winding of a transformer when a switching element turns off.
2. Description of the Related Art
FIG. 4 shows a circuit of an isolation type DC/DC converter of a general forward topology. In the figure, numeral 1 denotes a transformer whose primary and secondary sides are isolated from each other, numeral 2 denotes a switching element comprising, for example, MOSFET, and a series circuit of a primary winding 1A of the transformer 1 and the switching element 2 is coupled across a DC source 3 supplying DC input voltage Vi. A rectifier smoothing circuit 9 comprising a rectifier diode 5, a free-wheeling diode 6, a choke coil 7 and a smoothing capacitor 8 is coupled to a secondary winding 1B of the transformer 1. A voltage induced in the secondary winding 1B of the transformer 1 according to the switching operation of the switching element 2 is rectified and smoothed so that a DC output voltage Vo is developed at output terminals 10 and 11 coupled across the smoothing capacitor 8. In the circuit of the example, when the switching element 2 turns on, a positive voltage is induced at a dotted side terminal of the secondary winding 1B of the transformer 1 so that the rectifier diode 5 turns on and the free-wheeling diode 6 turns off. As a result, energy is fed through the rectifier diode 5 to the choke coil 7, and then to a load (not shown) between the output terminals 10 and 11. On the other hand, when the switching element 2 turns off, a positive voltage is, in turn, induced at a non-dotted side terminal of the secondary winding 1B so that the rectifier diode 5 turns off and the free-wheeling diode 6 turns on. Accordingly, in this case, energy which has been already stored in the choke coil 7 is fed to the load coupled between the terminals 10 and 11.
Numeral 12 denotes an amplifier circuit for monitoring output voltage, which monitors the DC output voltage Vo and feeds an amplified differential signal according to a difference from a reference voltage to a PWM (Pulse Width Modulation) controlling circuit 13. A pulse conduction width of a drive signal fed to a gate of the switching element 2 in response to the variation of the DC output voltage Vo can be varied for stabilizing the DC output voltage Vo.
FIG. 5 shows a voltage Vds across a drain and a source of the switching element 2 in FIG. 4, and a voltage Vr at a cathode whose reference point is set on an anode of the rectifier diode 5 in FIG. 4. In the figure, during the ON period (Ton) of the switching element 2, the DC input voltage Vi is applied across the primary winding 1A of the transformer 1 so that a voltage across the switching element 2, that is, the voltage Vds across the drain and the source becomes zero and due to a voltage induced at the dotted side terminal of the secondary winding 1B of the transformer 1, the voltage Vr across the rectifier diode 5 also becomes zero. (In this case, a forward voltage drop of the rectifier diode 5 is assumed to be negligible.)
On the other hand, during the OFF period (Toff) of the switching element 2, the reset voltage is developed across the primary winding 1A by an inertial current flowing through the primary winding 1A of the transformer 1 so that the voltage Vds across the drain and the source of the switching element 2 becomes one where the DC input voltage Vi from the DC source 3 and the reset voltage of the primary winding 1A are superimposed (S1 region in FIG. 5). At this time, since a voltage proportional to a turn ratio of the secondary winding 1B to the primary winding 1A is induced at the dotted side terminal in the secondary winding 1B of the transformer 1, the rectifier diode 5 turns off while the free-wheeling diode 6 turns on so that the voltage generated in the secondary winding 1B is directly applied to both terminals of the rectifier diode 5 (in this case, provided that the forward voltage drop of the rectifier diode 5 is assumed to be negligible).
When a core of the transformer 1 is reset soon, the reset voltage ceases to be generated in the primary winding 1A so that voltage Vds across the drain and the source of the switching element 2 becomes equal to the DC input voltage Vi, while the voltage Vr across the rectifier diode 5 becomes zero.
Thus, the ON-OFF operation of the switching element 2 causes magnetic flux passing through the core of the transformer 1 to repeat increase (set) and decrease (reset) alternately. However, in order to prevent the transformer 1 from being saturated due to incomplete reset, the DC input voltage Vi and a variable range of duty of the switching element 2 or the like are so determined that a product S1 of the voltage and the time during the OFF period of the switching element 2 is equal to a product S2 of those during the ON period of the switching element 2. However, if the ON period of the switching element 2 is prolonged by an abrupt change in an output power or the like, as such relation that S1 is equal to S2 is kept as aforementioned, the reset voltage of the transformer 1 during the OFF period of the switching element 2 increases due to the abovementioned prolongation of time, so that as shown by chain lines in FIG. 5, peak values of the voltage Vds across the drain and the source of the switching element 2 and those of the voltage across the rectifier diode 5 are increased. Therefore, such an element with an inferior property and a high withstand voltage that can withstand even during such increase in the reset voltage, had to be employed as the switching element 2 or the rectifier diode 5.
Accordingly, in view of the abovementioned problems, the present invention aims at providing a DC/DC converter that can control effectively increase in a reset voltage of a transformer during the OFF period of a switching element by using a simple circuitry.
The DC/DC converter of the present invention including a series circuit, of a primary winding of the transformer and the switching element, which is coupled to a DC source and a rectifier smoothing circuit which rectifies and smoothes a voltage induced by the switching operation of the switching element in a secondary winding of the transformer, wherein a Zener diode is coupled in parallel with the switching element with reverse polarity to the switching element.
Thus, during the OFF period of the switching element, a voltage, which is the sum of an input voltage of the DC source and the reset voltage generated in the primary winding of the transformer, is applied across the switching element. However, when the voltage across the switching element reaches an operating voltage of the Zener diode, the Zener diode conducts so that the reset voltage ceases increasing further. As a result, even if the ON period of the switching element is prolonged due to an abrupt change in an output power or the like, voltages both across the switching element and across the rectifier diode during the OFF period of the switching element are inevitably clamped whenever the Zener diode begins to conduct. Accordingly, notwithstanding such a simple circuitry that only the Zener diode is added to a conventional circuitry, it becomes possible that during the OFF period of the switching element, an increase in the reset voltage of the transformer can be effectively ceased so that an element with a superior property and a low withstand voltage can be employed as the switching element.